1. Field of the Invention
This invention relates to means for sensing liquid level and temperature and more specifically to an oil level and temperature sensor utilizing semiconductive material having a peak resistivity at a predetermined temperature.
2. Description of the Prior Art
It is frequently desirable to provide a device for automatically sensing the presence of a sufficient quantity of liquid in a container where the liquid may or may not be subject to heating.
The conventional thermal sensing circuits which exist in the prior art measure the change in thermal temperature of the liquid in the container. They will only respond, therefore, when evaporation occurs due to excessive heating of the liquid, while failing to respond to a drop in the liquid level that was not caused by excessive heating, such as a leak in the liquid container.
The thermal sensing circuit of Prussin, disclosed in U.S. Letters Pat. No. 3,412,610, overcomes partially the aforementioned problem. The Prussin device takes advantage of the fact that a liquid such as water is a far more effective coolant than air and if the thermal sensor in the circuit is subject to self-joule heating, its temperature will rise in the absence of the liquid coolant. Thus if the resistivity of the Prussin semi-conductive thermal sensor is such that it peaks at a given predetermined temperature, and if the temperature of the sensor exceeds the predetermined temperature at which peak resistivity occurs due to self-heating caused by the absence of liquid coolant, a runaway current results in the circuit due to the precipitous drop in resistivity of the sensor above the predetermined temperature, which in turn leads to more joule heating and a further drop in the resistivity. In short, a thermal and current avalanche is induced.
Particular problems are created however, if, first of all, the liquid, the level of which is to be sensed, is not a far more effective coolant than is air. If this is the case, it is evident that thermal and current avalanche may occur due to self-joule heating, regardless of whether the sensor is in contact with the liquid or not. Oil, for instance, is such a liquid. Secondly, if under normal operating conditions, the temperature of the liquid varies over a wide temperature range, then the liquid level sensor must be able to respond to a drop in liquid level which occurs at any point over this entire wide temperature range. The oil temperature in an internal combustion engine, for instance, normally varies over a temperature range from -50.degree. F to 300.degree. F (or -45.degree. C to 150.degree. C). Thus, an effective oil level sensor must be able to sense a drop in the oil level over this entire temperature range.
As noted above, peculiar problems and phenomena occur when the device of the Prussin patent is attempted to be used to monitor the level and temperature of a fluid which is not a good conductor of heat. Some of those problems will now be discussed in detail.
The primary problem is the inability of the fluid to conduct heat, which results in a build up of heat in the fluid immediately surrounding the sensor. This problem is particularly severe if the fluid is not circulating, as is the case when an automobile is parked. To illustrate this problem, consider the device of Prussin as it would operate with a fluid such as water, compared to a fluid such as oil. When the device of Prussin operates with water, the self-joule heating is readily absorbed by the water since water is a good thermal conductor, even if the water is non-circulating. The Prussin device may even work in hot non-circulating water. If the water is very hot and non-circulating, the self-joule heating of the Prussin device may cause the water in contact with the diaphragm, in the immediate vicinity of the semiconductor material, to boil. The low viscosity of water would permit the bubbles of gas produced by boiling to rapidly float away from the vicinity of the semiconductor, causing new water to be brought into contact with the hot surface of the diaphragm thereby cooling the device. The low viscosity and high thermal conductivity of the water combine and effectively cool the Prussin device even when the water is very hot and local boiling occurs near the sensor as a result of self-joule heating. It is to be noted that this is so, even though the semiconductor material of the prussin device is mounted on a diaphragm having a high thermal conductivity between the semiconductor material and the liquid and a low almost insulative thermal conductivity in the direction of the plane of the diaphragm. Thus, even though the Prussin device is somewhat thermally isolated upon the diaphragm, thereby permitting a local heat build-up due to joule-heating, the presence of even very hot non-circulating water is sufficient to cool the semiconductor material so that it does not reach a temperature corresponding to its peak resistivity. The above described operation of the Prussin device in water, or in radiator coolant, will now be contrasted with the manner in which it would perform in a liquid of greater viscosity and lower thermal conductivity, such as oil.
Consider first the operation of the device of the Prussin patent is immersed in cool oil. If the cool oil is sufficiently circulating, there will be little heat build-up in the vicinity of the semiconductor device, as the temperature difference between the oil and semiconductor device is sufficient to insure adequate cooling of the semiconductor. If the cool oil is non-circulating, as when the automobile is parked overnight, problems occur and prevent proper operation of the Prussin device. Since the cool oil is very viscous and a poor conductor of heat, the heat produced by joule-heating of the semiconductor device is not removed from the immediate vicinity of the semiconductor material. This heat builds up to the point where the viscous fluid may even begin to boil in the immediate area of the semiconductor material. Because the surrounding fluid is cool and viscous, the gases produced by the local boiling are trapped and further insulate the semiconductor material from the cooler oil. This heat build-up will continue to the point where the semiconductor material reaches its maximum resistivity and eventually triggers the trouble indicator (warning light, meter, etc.). This condition is further aggravated in the device of Prussin because that device is placed on a thin diaphragm the purpose and construction of which is to thermally insulate the semiconductor device from the walls of the container of fluid, the effect of which is to trap the heat build-up in the vicinity of the semiconductor material. The device of Prussin thus will not properly function in a non-circulating cool viscous liquid.
Consider second the operation of the device of the Prussin patent is immersed in heated oil. Based upon the discussion of the cool non-circulating oil it should be self evident that the Prussin device could not properly function in heated oil (or other viscous liquid). The temperature difference between the semiconductor material and heated oil is less than the temperature difference between the semiconductor material and cool oil, resulting in a net loss of cooling efficiency, even though the viscosity of the heated oil is less than the viscosity of the cool oil. Since the cooling efficiency of the heated oil is less than the cooling efficiency of the cool oil, the Prussin device cannot operate better than as in the cool non-circulating oil. Since it has been shown that the Prussin device will not operate properly in cool uncirculating oil, it will not operate it heated uncirculating oil.
The final environment to be examined for evaluation of the operation of the Prussin device is hot circulating oil (or other viscous fluid). Although the oil is relatively hot and therefore less viscous than cool oil, its cooling efficiency is still nearly as low as non-circulating hot oil due to the fact that all the circulating oil is uniformly hot. Even though the oil is circulating, a thin layer of hot oil tends to adhere to the surface of the diaphragm and this thin layer has a very low velocity. This low cooling efficiency of the hot circulating oil combines with another feature of the Prussin device, namely the thin diaphragm which tends to insulate the semiconductor material from the wall of the liquid container but not from the liquid, to permit a build-up of heat in the thin layer of relative static hot oil adhering to the diaphragm. The heat build-up is not dissipated to the container walls and continues to build up to the point where either (1) the thin layer of adhering oil begins to boil, further insulating the semiconductor material from any cooling effects of the circulating hot oil, thus causing the semiconductor material to indicate overheated oil when in fact the oil is not overheated or (2) even if the thin layer of oil does not begin to boil, the oil is unable to cool the semiconductor sufficiently to prevent it from reaching its peak resistivity. The self-joule heating of the Prussin device, when used in a hot and viscous fluid, is too high to permit proper operation of that device. The preceeding discussion has to be qualified somewhat to allow for the fact that if the velocity of oil is sufficiently high, the Prussin device might operate satisfactorily, depending on the velocity of oil, but would not operate satisfactorily over a broad velocity range. The operation of the Prussin device is highly dependent on the velocity of circulating hot oil. The identification of the nature of the problem was only the first step in effecting a solution. Basically, the problem with the Prussin device is that it relies exclusively upon the fluid to dissipate the self-joule heat generated by the semiconductor material. This is made manifest at column 4 line 73 to column 5 line 3, where it is stated that "the temperature of the senor is substantially unaffected by that of the wall of the container".
It is thus an object of the present invention to construct a device for sensing the presence or absence of fluid at a given level in a container and sensing whether that fluid is above or below a predetermined temperature which device is functional in fluids of low viscosity and high thermal conductivity such as water and relatively high viscosity and low thermal conductivty such as motor oil.
It is a further objective of the present invention to provide an oil level sensor which operates in a thermal circuit with a 12-v. battery and with a small light bulb as the only load and current limiting device.